The New York City subway system is becoming more delay-prone, according to a new analysis released this morning.

Every year starting in 2011, a riders advocacy group called the Straphangers Campaign analyzes the electronic alerts sent by the M.T.A. to subway riders who subscribe to the service.

The M.T.A. sends out alerts warning straphangers of "significant" delays when an incident occurs that the agency believes will hold up a train for more than 8 or 10 minutes.

In 2011, the agency sent out those sorts of alerts for "controllable" incidents (those not involving circumstances like sick passengers and police investigations) 2,967 times. In 2013, the agency sent out such alerts 3,998 times. That's a 35 percent jump. (2012 was excluded from the analysis because of Hurricane Sandy.)

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“The increase in alerts is a troubling sign that subway service is deteriorating,” said Gene Russianoff, staff attorney for the Straphangers Campaign, in a statement.

Signal and mechanical problems generate more than two-thirds of those alerts, and the F train, by constituting 8 percent of the total, is the worst offender. The 4, 2, 5, N and D trains followed close behind, with each generating 7 percent of all alerts.

By that measure, the best performing subway was the J/Z line.

The M.T.A. had no immediate comment, but you can read the whole report here.

Update: In a statement, the M.T.A. contested the Straphangers' findings.

"Since 2011, the amount of time customers have had to wait for a train throughout the system has remained flat," said Kevin Ortiz, an M.T.A. spokesman. “We agree that the service alerts are a powerful tool that deliver meaningful information to customers. We have increased staff and have become more efficient in providing service information in a more timely manner so customers are quickly aware of any incidents that may impact their commute. However, the cause of such incidents can quickly change upon further investigation which is why the alerts were never meant to serve as a performance metric. Our wait assessment metric, which includes BOTH controllable and non-controllable incidents and measures the amount of time customers have had to wait for a train, provides a more comprehensive picture of service quality."